An EL device utilizing electroluminescence has a high degree of visibility due to the self-emitting nature thereof. In addition, being a perfect solid device, it has benefits such as excellent impact resistance. For these reasons, use of an EL device as an emitting device in various displays has attracted attention.
The EL device is divided into an inorganic EL device using an inorganic compound as an emitting material, and an organic EL device using an organic compound as an emitting material. Of these, an organic EL device has been developed as the next-generation emitting device, since it can significantly reduce an applied voltage, can easily attain full-color display, consumes only a small amount of power and is capable of performing plane emission.
Although an organic EL device basically comprises an anode, an organic emitting layer and a cathode being stacked in this order, various device configurations are studied with the aim of developing an organic EL device having a high efficiency and a long life.
In order to enhance the efficiency and prolong the life, a technology has been disclosed in which a plurality of units are stacked, with each unit consisting of a cathode, an organic emitting layer and an anode being stacked in this order (see Patent Documents 1 to 3, for example). As compared with a single-layer device, this organic EL device can have a prolonged life, since only a lower current density is required to obtain a luminance which is equivalent to that attained by the single-layer device. However, the above-mentioned technologies have such a disadvantage that a driving voltage is significantly increased, since a plurality of devices is stacked in series.
Patent Document 1: JP-A-06-176870
Patent Document 2: JP-A-11-312584
Patent Document 3: JP-A-11-312585
The invention has been made in view of the above-mentioned problems, and the object thereof is to provide an organic EL device which has a high degree of efficiency and can be operated at a low driving voltage even though a plurality of emitting layers are stacked in series.